l. Field of the Invention
The present invention is directed to a process for the manufacture of articles of translucent alumina by sintering of molded articles from an alumina powder containing a small amount of a grain growth inhibiting substance, such as magnesia, or a salt of magnesium, or an equivalent metal, which is decomposable by heating with formation of magnesia, or an equivalent metal oxide, which comprises preliminarily sintering, in an oxidizing atmosphere, at a temperature above 700.degree. C followed by a final sintering at a temperature above 1700.degree. C in a non-oxidizing medium.
2. Description of the Prior Art
According to U.S. Pat. No. 3,026,210, articles of translucent alumina in the visible and near infra-red spectra are prepared by a process similar to that of the present invention, in which the molded articles, consisting of alumina containing from one-sixteenth % to one-half % of magnesia, are subjected to a preliminary sintering step at a temperature range of between 1000.degree. C and 1200.degree. C, and then, further subjected to a final sintering step at a temperature between 1800.degree. C and 1950.degree. C in the presence of a hydrogen atmosphere, or under a vacuum, the preliminary sintering step sometimes being omitted. Such sintering steps are normally carried out in furnaces, the interior surface and the inner articles of which are generally composed of carbon, preferably graphite, because of its refractoriness, chemical inertia, relatively low cost and ease of manufacture, even with respect to large articles out of the same material. However, a sintering of alumina articles in a carbon furnace necessitates the employment of an atmosphere consisting of extremely pure hydrogen or of a very high vacuum to avoid large scale transfers of alumina from the articles to be sintered onto the walls and inner articles of the furnace.
While the above-mentioned patent employs a process similar to that of the present invention, it should be emphasized that the process disclosed therein is only operative for making transparent alumina bodies, when the final sintering step is carried out in an atmosphere of hydrogen or under vacuum. See for instance, U.S. Pat. Nos. 3,026,177 and 3,331,482. Because of the manner in which the process of U.S. Pat. No. 3,026,210 is carried out, extreme inferior transparency of the final product is observed. To the contrary, with the process of the present invention, as described below, transparency values approximating 93 to 94% in the visible spectrum are obtained.
U.S. Pat. No. 3,454,358 discloses the production of infra-red transparent bodies of zinc selenide, wherein a non-oxidizing, heat treating atmosphere, such as argon is employed to avoid roasting of the zinc selenide into zinc oxide and selenium dioxide.
Lastly, French Pat. No. 852,178 discloses sintering bodies of alumina (without any concern or disclosure of its purity), other metallic oxides and mixtures of metallic oxides and carbides in a carbon monoxide atmosphere; however, the process described therein relates to an art, quite divergent from that of the present invention (i.e., cutting tools) in which there is, of course, no problem in providing transparent bodies.